C3 Driveability Improvement - DIY EFI

How one novice tuner improved the driveability of his C3 for less than $1,000

Nothing beats the prospect of a long, enjoyable drive in your Corvette--except, perhaps, an out-of-tune engine that stumbles, stalls, or generally falls short in the driveability department. We're talking mostly about the smog-laden carbureted C3 models of the mid-to-late '70s. These Quadrajet-fed small-blocks perform well enough when tuned properly and driven regularly, but if your Corvette is a fair-weather cruiser and sits for several months during the winter, that Q-jet can become a source of frustration and unreliability. Such was the case with a friend's '77 Corvette that was driven infrequently, mostly because of a gummed-up carb that made even quick drives a testy, smelly affair. That friend--Leonard Slebodnik--thought about having the Quadrajet rebuilt, but admitted he'd really prefer the driveability advantage of electronic fuel injection. "I really wanted the assurance that when I jumped behind the wheel, the engine would fire up and idle correctly every time," he told us. "A new or rebuilt carb only seemed like a temporary fix for me."

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And while there are various injection conversion systems on the market, Slebodnik is a do-it-yourself kind of guy, and he wanted to see if he could build his own setup from scratch for less than $1,500. (A Q-jet rebuild is typically $400-$500, by the way.) It would involve some parts scrounging, an electronic controller for the system, and a bit of welding. Slebodnik is in no way a professional tuner--and this project represented his first time ever tackling air/fuel tables on a laptop--but he is blessed with a diagnostically inclined brain, which proved to be a big help.

"If I could do it, just about anyone could," he says. "The physical conversion of the parts is pretty straightforward, but you have to be really careful and diligent with setting up the tables with the controller software--and you should have a basic understanding of what the engine needs for proper operation."

The homemade system is directed by the venerable MegaSquirt EFI controller, which we've seen on everything from budget conversions, such as Slebodnik's, to mega-dollar drag-race combinations. It's intended for stand-alone systems, driving only the engine's injection system. It's also designed to be used with a speed-density air-metering system, so it should work with the GM TBI unit Slebodnik planned to use.

And here's the kicker of the project: It was done for only about $900. Seriously.

Collecting parts and getting dirty

Before diving into this project, Slebodnik spent considerable time on the Internet researching it. He found sites and forums--specifically megamanual.com and mesfi.com--where others who had attempted the same basic conversion offered advice on parts, installation, and tuning. With a materials list in hand, he started scouring auto parts stores, the Web, and a few salvage yards for the components. The basics included:

Removing the original Q-jet and replacing it with the throttle-body apparatus is a comparative snap. The project gets more involved when it comes to routing and connecting the necessary fuel-return line from the throttle-body system to the fuel tank. Although the tank itself doesn't require modification, the removable section that contains the fuel-fill inlet and sending unit does, including a bit of welding. It also means removing the fuel tank, as well as bolting the new high-pressure fuel pump beneath it after reinstallation. Finally, a bung for the wide-band oxygen sensor will need to be welded into the exhaust system.

As for the MegaSquirt controller, it found a home beneath the forward portion of the center console, which provided reasonably easy access for plugging in a laptop for tuning. Room under the hood must also be made for the relay box that supports the injectors and fuel pump.

Tuning trials and tribulations

While Slebodnik had never attempted tuning a car before, he's an IT guy by profession, so he knows his way around a keyboard.

"Having never attempted anything like tuning for fuel injection before, it was a challenge," he says. "Even though I knew how an engine worked, and the basics of air/fuel ratios, and so on, I was still starting from zero when it came to getting the car started."

Slebodnik points to the megamanual.com Website and associated forums as an invaluable resource. They provide the programming basics needed to get the engine started and idling. After that, the trial-and-error fine-tuning process begins.

"One of the hardest things to learn at the beginning was setting the 'constants'--things like the required fuel, injector flow rate, required fuel under acceleration, and the targeted stoichiometric values [optimal air/fuel ratio]," he says. "With those set, you can get the car running, but the tuning variables are almost infinite, so for the first-timer, there's a lot of trial and error. Fortunately, you can save a 'tune,' and if it doesn't work as well as a previous one, it's easy to switch back."

As we were heading toward the deadline on this story, Slebodnik admitted that the cold-start enrichment tuning--which ensures that the engine has more fuel at start-up, until it reaches operating temperature--was still an issue to overcome.

"Yeah, it's still a bit cold-blooded at start-up, but I'm working on it," he says. "Once that's nailed, it will all be good. As it stands, it already runs much better than with the old carburetor."

Greater power and fuel mileage, right? Well…

Right off the bat, we'll address a couple of common misperceptions about converting to EFI. Generally speaking, don't expect huge increases in either performance or fuel economy on an otherwise stock engine. The injection system may foster more-efficient combustion, but that doesn't necessarily translate into noticeably more horsepower. You're looking at maybe a 5 percent increase in output, tops.

There are a couple reasons for this, the most notable being the fact that changing to fuel injection doesn't alter the airflow aspects of the intake manifold or cylinder heads. Those were pretty low-flow elements on '70s-era GM small-blocks, as engineers tried to squeeze more mileage and emissions efficiency out of them. So, electronically controlled fuel delivery or not, sticking with the stock intake and heads compromises performance. It's also one of the reasons this project uses a "two-barrel"-style throttle-body injection system rather than a "four-barrel"-style. There's simply no need for the latter on an engine rated at only 180 hp.

Of course, fuel-injected performance took a large leap in 1985, when the Corvette debuted Tuned Port Injection. That swap is entirely possible on a C3, too, but it's a more costly endeavor and drastically alters the underhood appearance. Slebodnik wanted to retain the look of a carbureted engine, and the throttle-body injection system he selected tucks nicely beneath a conventional air cleaner.

As for the fuel economy, again, the car's stock configuration is the compromise--this time in the guise of its three-speed automatic transmission. Sure, there were a few manual-transmission models sold in the later C3 years, but even they didn't offer the advantage of overdrive. Be it a three-speed slushbox or the Borg Warner T10-based four-speed, the top-gear ratio was 1.00:1. Combine that with the typical 3.55 rear axle of the day, and highway cruising can yield 2,500 rpm or greater. Regardless of whether the engine is fuel injected, it's still going to suck gas pretty fiercely on the open road.

The bottom line

Although Slebodnik is still revising his tune to nail down the cold-start performance, he's done spending money for parts on the system. The total--itemized in the accompanying chart--came to $904. That, of course, doesn't include the sweat equity of doing the installation and tuning himself, but the satisfaction of the project suggests the labor wasn't much of a factor for this project.

"There are a couple things I'd do differently, but I'd definitely do this again on another vehicle," he says. "Even with a couple of tuning issues to work on still, the driveability of the car is already much better than with the carburetor. I wouldn't go back to that again for anything."

Slebodnik says the experience has also given him a new appreciation and greater understanding of the complexities of modern engines, and why tuning is such a painstaking, specialized process.

Spoken like a true convert.

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C3 Driveability Improvement - DIY EFI

1 Let’s dive right in with the removal of the original Quadrajet. As is the case with many Corvettes that aren’t used as daily transportation—and are stored during the winter—this one fell short from a driveability standpoint. A desire to cure hesitation and cold-start problems, rather than a yen for greater performance, had the car’s owner chasing the fuel-injection solution.

2 While the ’77 L48 Corvette’s cast-iron intake manifold was suitable for adapting the throttle-body injection system, owner Leonard Slebodnik picked an aluminum intake from an ’80s Monte Carlo SS off the shelf of a buddy’s garage, where it had been collecting dust for 20 years. A quick clean-up and rattle-can re-spray to match the ’77 engine color delivered an undetectable enhancement that took about 10 pounds off the top of the engine. Nice.

3 You’ll need an adapter plate to mount the two-barrel-type TBI unit to the four-barrel manifold. This is required whether you’re working with the stock Q-jet–type spread-bore design or an aftermarket intake with a Holley-style square-bore design. Note the counter-sunk fasteners at the rear of the adapter plate. They’re necessary to provide adequate clearance of the TBI unit. This plate is easy to make, but aftermarket plates are available for $50-$75.

4 The TBI unit used for the project is simply a salvage- yard part scavenged from any of the millions of GM trucks built from the mid ’80s through the mid ’90s—including the 4.3L V-6, the 5.0L and 5.7L small-blocks, and even the 7.4L big-block (the latter will require a larger-bore adapter plate). This one came from a 4.3L S-10, featured a pair of 45-lb/hr injectors, and was purchased at a pull-your-own yard for $35. It’s important to make sure the unit still has the throttle-position sensor (TPS) and idle-air control sensor (IAC). A couple of coolant sensors are required, too, so be sure to grab them from the same donor engine. For safety’s sake, it’s also a good idea to replace the TBI unit’s gaskets and seals.

5 A 3⁄8-in fuel line rated for high-pressure fuel injection is required to feed the TBI unit. That means running all-new line from the tank to the TBI. Slebodnik grabbed some of the flexible hard-line shielding from the salvage-yard engine.

6 An EFI system requires a return fuel line that’s close in diameter to the feed line. In our case the original 3⁄8-inch feed line was of the correct size and in good shape, so we used it as the return.

7 To accommodate the larger-diameter return line, this 5⁄16-inch fitting (arrow) on the fuel-tank sender unit (located at the top of the fuel tank) must be cut off and replaced with a 3⁄8-inch fitting that is TIG welded to the sender. The sender is easy to unbolt from the tank, but it can get a bit grungy after 35 years. An alternative is starting with a clean, all-new replacement. This one is from Mid America Motorworks and costs about $170.

8 When it comes to sending fuel to the engine, the original block-mounted mechanical pump must be replaced with a high-pressure electric unit. (Don’t forget the block-off plate for the stocker.) Slebodnik used an external pump and filter for a late-’80s/early-’90s Ford F-250 with a TBI 7.5L (460) engine. The GM TBI runs with about 12-15 psi and features an internal pressure regulator. But even with this electric pump, it still needs gravity feed, so the pump must be mounted below the tank. The wiring must also be run to a relay under the hood, so be sure to keep it away from the exhaust.

9 The TBI unit’s throttle mechanism is positioned closer to the firewall, necessitating a new, adjustable linkage bracket (arrow). It’s available from outlets like Jegs and Summit Racing for only a few bucks and it simply mounts to the intake manifold.

10 The MegaSquirt components came from diyautotune.com, the parts’ primary manufacturer and distributor. And while some very basic, stripped-down components for the system are available for a few dollars less, purchasing a pre-assembled relay cable and pre-labeled wiring kit proved to be a better value—especially for a first-timer. All together, the ECM, stimulator board, relay board, relay cable, and wiring kit cost just $387.

11 The stimulator board must be carefully assembled by soldering the connections. Be sure to check the connections periodically during the assembly process. This way, if any problems arise, you can trace them more easily.

12 After assembling the board, it should be washed with acetone to remove any soldering flux that could cause “bridging” problems. Then, the board should be sprayed with a clear lacquer to seal it. That will make it more reliable.

13 There’s no getting around drilling a hole in the firewall, since the harness for the controller is mounted inside the car and must be connected to the relay board under the hood. There’s also a vacuum line to the MegaSquirt controller for the MAP sensor that’s mounted on its board. This requires drilling a 1-inch hole through the fiberglass body, near the top of the gas-pedal area.

14 There’s room to mount the controller under the forward section of the center console. It’s a tight fit, but an unobtrusive one in the otherwise tight confines of the C3 cabin. The unit should be positioned so that the serial port for the laptop hookup is on the outside for quick access. It’s also easy to run a length of cable from the controller under the console to a discreet location near the rear of the seats for an easier plug-in. The MegaSquirt II and other controllers are starting to incorporate USB ports, too.

15 The complementing relay board for the control system mounts under the hood. The best location is at the upper-rear corner, on the left side of the engine compartment. The three large relays mounted on the board are for the fuel pump (top), injector No. 2 (center) and injector No. 1 (bottom). The injectors used separate relays because they’re pulsed (triggered) separately.

16 One of the recommended options with the MegaSquirt system is the pre-labeled wiring kit, which includes automotive-grade wiring tagged with where each wire should go. It makes installation infinitely easier and a lot faster. Even so, plan on at least six hours to wire up the engine. Use heat-shrink connectors, too, to ensure weatherproof connections.

17 With the wiring completed and tucked inside the covers, the appearance is tidy and straightforward. As seen here, the TPS, IAC and coolant-temp sensors are connected. What’s not visible is the necessary air-intake sensor, which mounts to the bottom of the air cleaner. A coolant-temp sensor can be used for this (that’s why Slebodnik grabbed two at the salvage yard), and installation is as simple as drilling a small hole in the base of the air cleaner and threading in the sensor.

18 The final piece to install is a wide-band oxygen sensor, which provides the engine controller with specific feedback on the oxygen content of the exhaust gas (rather than the simple lean-or-rich feedback of a narrow-band sensor). This enables the controller to adjust fuel and—on the MegaSquirt II and other controllers—ignition timing more precisely. (The basic MegaSquirt used here doesn’t allow for easy timing adjustment.) The sensor is mounted in the exhaust system, where the Y-pipe merges with the single outlet pipe.

19 After drilling a pilot hole and a corresponding large hole, a bung for the oxygen sensor is welded to the Y-pipe. The homemade welding job ain’t pretty, but it’s all that’s needed to mount the sensor and connect it to the wiring harness. Ideally, the sensor should be mounted at the top of the exhaust, ahead of the catalytic converter. This isn’t a problem for the late C3s, which have adequate clearance. Wiring for the sensor is routed to the controller.

20 Although the installation of the TBI system was relatively straightforward, the tuning process can be daunting to the uninitiated. There’s no two ways about it: There’s going to be a trial-and-error adjustment process after a baseline program is established. Owner/builder Leonard Slebodnik reports that the most difficult aspect is nailing the cold-start enrichment.

21 Here’s a look at the finished installation, which would have looked completely factory had Slebodnik reinstalled the original air cleaner. The adapter plate and TBI unit combine for a taller installed height than the original Q-jet, so hood clearance can be an issue. Slebodnik used an aftermarket air cleaner with a forward-offset base to mount the air-temp sensor, but he found that the standard 3-inch-high filter element was too tall. A two-inch element did the trick, but it’s still a tight fit.

22 The lack of a nearby chassis-dyno facility in Slebodnik’s rural hometown prevented us from securing before-and-after testing of the project car, but the seat-of-the-pants evaluation demonstrated excellent driveability, with crisp throttle response and no hesitation, stumbling, or other issues experienced with the tired original carburetor. Not bad for under a thousand bucks.